Disable shellcheck SC1090

We use `REPO_DIR` in file paths when sourcing so shellcheck cannot find
the files. Disable the lint.

(I thought -x used to shoosh this warning but it doesn't seem to be
doing so.)

ci/run_task.sh

@@ -1,6 +1,9 @@
 #!/usr/bin/env bash
 #
 # Script used to run CI jobs, can also be used from the command line.
+#
+# disable can't follow non-constant source (because paths use $REPO_DIR).
+# shellcheck disable=SC1090
 
 set -euox pipefail